Thank you for the answer. Last night I made a test: I exposed the same picture part to the same paper with 3 different lenses: with Belar 50/4.5, with SMC Takumar 55/1.8, and with Helios 58/2 (old type, with 39 mm lens mount). Both lenses was stopped down to f/8. The developed paper showed the following result: The Takumar is the best in this comparison, it has the sharpest picture. The difference between the Belar and the Takumar is pretty big. The Helios is very good, more better than the Belar, but the difference between the Takumar and the Helios is very small. It is possible that I buy a Scheneider Kreuznach Componar lens from ebay, if it is arrives I will retest the lens, but 4 instead of the 3 above.